The present invention is directed to polyimide resins having improved thermo-oxidative stability. The polyimide resins are generally useful in applications where high strength and temperature capabilities are required at temperatures up to about 700.degree. F. Polyimide resins have been particularly useful in applications such as jet engine compressor components, for example, blades, vanes, air seals, air splitters, and engine casing parts.
Polyimides having excellent high temperature oxidative and thermal stability and outstanding mechanical properties are described in U.S. Pat. No. 3,745,149 to Serafini et al. These polyimides, also referred to as PMR (in situ polymerization of monomer reactants) polyimides, are the most widely used addition-type polyimide matrix resins for aerospace applications requiring use temperatures of 450.degree. F. and higher.
Two versions of PMR polyimide resins are referred to as first and second generation PMR's (PMR I and PMR II respectively). These addition-type polyimides comprise monomer components of diester, diamine and an end-capper in a stoichiometric ratio of n moles of diester, n+1 moles of diamine and 2 moles of the end-capper. The first generation PMR polyimides comprise the monomethyl ester of 5-norbornene-2,3-dicarboxylic acid, the dimethyl ester of 3,3',4,4'-benzophenonetetracarboxylic acid and 4,4'-methylenedianiline. The monomers used for second generation PMR polyimides are the monomethyl ester of 5-norbornene-2,3-dicarboxylic acid, the dimethyl ester of 4,4'-(hexafluoroisopropylidene-bis(phthalic acid)), and p-phenylenediamine. These polyimides both employ the monomethyl ester of 5-norbornene-2,3-dicarboxylic acid as the end-capper, but differ in the diester and diamine used to prepare the resin. See U.S. Pat. No. 3,745,149.
Prior investigations show that the 371.degree. C. oxidative stability of PMR and PMR II resin formulations improves as the molecular weight is increased. But this improvement reaches a plateau at a molecular weight of 5000. See Vannucci, "PMR Polyimide Compositions for Improved Performance at 371.degree. C.", 32nd Int'l SAMPE Symposium and Exhibition, April 1987. In addition, high molecular weight (HMW) PMR polyimides require higher curing temperatures and higher pressures than low molecular weight polyimides.
Melting temperature and melt viscosity are related to a polymer's molecular weight. Accordingly, the resin flow of HMW PMR polyimides is restricted during processing. As a result, low molecular weight prepolymers are easier to process, but lack the excellent thermal oxidative stability provided by HMW polyimides.
One type of PMR polyimide is disclosed in U.S. Pat. No. 4,560,742 to Pater. The disclosed polyimides are derived from a dialkyl or tetraalkyl ester of an aromatic tetracarboxylic acid, an aromatic diamine, an end-cap of a monoalkyl or dialkyl ester of a dicarboxylic acid, and an N-arylnadimide such as N-phenylnadimide. Graphite composites may be formed from the polyimide resins having a life of 1700 hours at 600.degree. F. in air.
Low curing temperature PMR polyimides are disclosed by Serafini et al, "Lower-Curing-Temperature PMR Polyimides", NASA Technical Memorandum 81705, Thirty-sixth Annual Conference of the Reinforced Plastics Composites Institute of the Society of the Plastics Industry, Inc., 1981. An endcap of m-aminostyrene was used for reducing the cure temperature requirements of PMR polyimides. The use temperature of these composites, however, is limited to about 260.degree. C. (500.degree. F.).
U.S. Pat. No. 4,739,030 to Lubowitz et al discloses difunctional end-capped monomers, which can be linked to form thermoset polymers.
Polyesters containing fluorinated units are known. U.S. Pat. No. 4,433,132 to Rodgers et al discloses thermoplastic polyesters containing 2,2'-bis(trifluoromethyl)-biphenylene units. The resultant polyesters are said to exhibit favorable solubility properties and are useful for producing films and fibers.
While the above patents are directed to resins having certain desirable properties, there continues to be a need for higher molecular weight polyimide resins having improved properties, and in particular, having improved thermo-oxidative stability and easy processing.